The present invention relates to a process for vitrifying liquid radioactive waste, and more particularly a process suitable for the solidification of high-level liquid radioactive waste.
Slightly enriched uranium nuclear fuel used in a light water reactor of a nuclear power plant is reprocessed to recover unburned .sup.235 U and formed .sup.239 Pu in a spent fuel.
In this treatment, in general, the spent fuel is dissolved in nitric acid to recover U and Pu by a wet process called the purex process and thereafter remains quite high-level liquid radioactive waste containing at least 99% of fission products (hereinafter referred to as "FP") and a small amount of actinides such as .sup.241 Am and .sup.244 Cm.
The liquid waste has a radioactivity of at least 10.sup.6 Ci per ton of the uranium fuel and contains at least 1 Ci of radioactive substances per ml of the liquid.
Further, FP and the antinides emit a radiation harmful to human bodies for a period of as long as 10.sup.3 years and 10.sup.6 years, respectively. Therefore, they should be isolated from the human life environment during these
The liquid waste obtained in the purex process is an approximately 2N nitric acid solution containing, in addition to FP and the actinides, corrosion products formed in the course of the above-mentioned process as well as suspended materials and precipitates. In addition, the solution generates heat, i.e., that of decay of the radioactive elements and, therefore, it is stored in a stainless steel tank having stirring and cooling means.
However, it is quite dangerous to store the liquid for a long period of time as described above. The Atomic Energy Commissionof Japan proposed that the liquid waste should be solidified in a stable form, stored for a while and then isolated from the human life environment for disposal.
Also in other countries having advanced nuclear power technologies, methods of solidifying the liquid waste have been developed. For example, in France, plant for the vitrification of liquid radioactive waste is now operated on an industrial scale.
In Japan, too, investigations have been made on the solidification processes of high-level liquid waste, mainly using borosilicate glass which is a leading solidifying medium in the world and the practical utilization thereof in the near future is expected.
Known solidification processes wherein borosilicate glass is used include (1) a two-step metallic melter process, (2) a two-step ceramic melter process and (3) a one-step ceramic melter method.
The two-step metallic melter process (1) comprises dehydrating, denitrating and calcining high-level liquid radioactive waste in a rotary kiln, or by a spray calcination method or fluidized bed method to form powder, mixing the powder with glass frit, feeding the resulting mixture in a metallic melter, heating the melter by, for example, a high-frequency induction method to melt the glass and cooling the glass for solidification.
The two-step ceramic melter process (2) comprises enriching, denitrating and calcining high-level liquid radioactive waste to obtain powder, mixing the powder with glass frit, feeding the resulting mixture in a glass-melting tank made of ceramic fire brick, heating mixture to a temperature at which electricity can be turned on, inserting electrodes into the molten glass to turn on electricity, melting the glass material by Joule's heat generated and cooling the molten glass for solidification.
The one-step ceramic melter process (3) comprises mixing high-level liquid radioactive waste with glass frit, feeding the resulting mixture in the form of a slurry in a glass-melting tank and then effecting the same treatment as in the above two-step ceramic melter process.
Technical studies are now made on a so-called one-step metallic melter process which comprises mixing high-level liquid radioactive waste with glass frit, feeding the resulting mixture in the form of a slurry in a metallic melter and then effecting the same treatment as in the above two-step metallic melter process, in addition to the above-mentioned liquid waste treatment methods (1) to (3).
However, these processes have many defects. For example, in the two-step metallic melter process (1) and two-step ceramic melter process (2), complicated two steps are necessary for the production and the liquid waste to be treated is generally an approximately 2N nitric acid solution and, therefore, troubles such as blocking of pipes, formation of deposit on the vessel walls, corrosion of the vessel walls or the like caused by the complicated steps occur inevitably in the course of the dehydration, denitration and calcination to obtain the powder.
In the newly developed one-step ceramic melter process and the one-step ceramic melter process (3), ebullition of water abruptly occurs due to the high temperature in the step of melting the glass material in a metallic melter or ceramic melter and, entrained materials such as liquid waste are scattered about in all directions. Furthermore, the metallic melter or ceramic melter is seriously damaged by a great temperature difference.
In all of the above processes, it is required that the vitrified product has an excellent chemical resistance, high homogeneity and freedom from the molybdenum phase separation. To satisfy these requirements, it is necessary to use high-silica glass frit. However, such glass frit has a melting temperature far higher than that having a low silica content. Accordingly, the melting and solidification of the glass in the former case must be effected at a temperature higher than that in the latter case. Consequently, problems are posed in the operation and it becomes inevitable that the metallic melter or ceramic melter is damaged and that the volatilization of ruthenium or cesium is enhanced.